Microsoft Corp. has released three software updates that patch critical security flaws in its products, including a patch for an Internet Explorer vulnerability that was first reported last week. The company also released patches for Microsoft Word and for a feature of the Windows operating system that is used by a number of applications.

All three of the patches, which Microsoft calls "updates," are rated "critical," meaning that the flaws they fix could allow malicious code to be installed on a user's computer with very little user action. The updates affect current versions of Windows and Internet Explorer as well as certain older versions of Word, according to Stephen Toulouse, security program manager with Microsoft's security response center.

The Internet Explorer (IE) and Windows patches appear to be the most significant, as the flaws they address could both be used by an attacker to take control of a user's system via a maliciously encoded Web page, said Neel Mehta, team leader of X-Force research with security vendor Internet Security Systems Inc. (ISS). The IE bug is significant because security experts have already shown a way that it could be exploited by an attacker, he said.

Last week, Microsoft issued a work-around to this problem, which concerns a file used by IE called Javaprxy.dll. However, Tuesday's patch fixes the underlying problem, Metha said.

ISS is also concerned about the Windows vulnerability, which relates to a feature called the Microsoft Color Management Module. This software is used to ensure that colors look the same when they are being rendered on different types of hardware, and is employed by a number of widely used applications, including Microsoft Outlook and IE, Metha said.

"Our initial analysis shows it being pretty conducive to exploitation," Metha said. "Any application that uses the built-in Windows facilities to show JPEG images, or possibly some other images, could be an attack vector for this vulnerability."

In fact, Microsoft has already privately been made aware of exploits of this flaw, Toulouse said.

The Word vulnerability, which could allow an attacker to gain control of a user's system when a maliciously encoded Word document is opened, does not affect the most recent version of the word processor. However, users of Word 2000, 2002 will need to install the patch, Toulouse said.

The three patches are detailed in Microsoft Security Bulletins MS05-35, MS05-36 and MS05-37. A new version of a previously released bulletin, entitled MS05-33 was also released Tuesday after Microsoft discovered that the Windows bug that it addresses also affects the company's Services for Unix 2.0 and 2.1. products, Toulouse said.

All three of the patches will probably require a reboot in order to take effect, Toulouse said. "If the files are in use when the update is applied, and in these cases they're pretty much going to be, that is what forces a reboot," he said.